Friday, November 4, 2011

URINARY TRACT INFECTIONS

Samples of urine from patients with suspected infections of the urinary tract are the most numerous, e.g. 30-40%. of the different kinds of specimens received in most clinical labora¬tories. The schedule for their routine examin¬ation should therefore be carefully determined with a view to obtaining the necessary diagnostic information with the greatest possible economy of labor and resources.
The examinations generally made are the microscopical examination of a wet film of uncentrifuged urine to determine whether poly¬morphs (‘pus cells’) are present in numbers indicative of infection in the urinary tract, and the semi-quantitative culture of the urine to determine whether it contains a potentially patho¬genic bacterium in numbers sufficient to ident¬ify it as the causal infecting organism (‘significant bacteriuria’).
The chemotherapy of a proven infection may be guided by in-vitro sensitivity tests on the pathogen isolated in culture and the outcome of therapy assessed by examination of the urine at the conclusion of treatment. Follow-up examin¬ation of patients who have had urinary tract infection is advisable because a relapse may be clinically silent.
The common symptoms of urinary tract infect¬ion are urgency and frequency of micturition, with associated discomfort or pain. The commonest condition is cystitis, due to infection of the bladder with a uropathogenic bacterium, which most frequently is Escherichia coli but sometimes Staphylococcus saprophyticus or, especially in hospital-acquired infections. Klebsiella pneumoniae var. aerogenes or oxyloca, Proteus mirabilis, other coliforms and Pseudomonas aeruginosa or Streptococcus faecalis. Candida infection may occur in diabetic and immuno-compromised patients. Rarer infecting organisms include Streptococcus agulacziae, Streptococcus milleri, and other streptococci, anaerobic streptococci and Gardenella vaginalis. There has been much debate on the significance of so-called fastidious organisms in urinary tract infection.
More serious bacterial infections are acute pyelitis and pyelonephritis in which the symptoms usually include loin pain and fever and which may be accompanied by a bacteriaemia detectable by blood culture. The causative organism may be any of those that cause cystitis, but Staphylococcus aureus is responsible for some of the cases.
Patients with signs or symptoms of urinary tract infection sometimes produce samples of urine that show pus cells but do not yield a significant growth of bacteria on routine culture.
The explanation may be that the patient has been taking antibiotics prescribed on a previous, occasion. Alternatively, he may have an infection with an organism that does not grow’ on the media normally used for routine investigations. In such cases it is important to consider genitourinary tuberculosis or gonococcal infection and infection with nutrition¬ally exacting or anaerobic bacteria. But many patients with frequency and dysuria do not have a bacterial infection of the bladder, nor significant numbers of bacteria in their urine (abacterial pyuria). Their condition is known as non-bacterial urethritis or cystitis, or the urethral syndrome. The cause of which may be urethral or bladder infection with a chlam¬ydia, ureaplasma, trichomonas or virus, which often remains unrecognized.
Screening out negatives
About 70-80% of the urine specimens received in a clinical laboratory are found on full microscopical and cultural examinations to be free from evidence of infection in the urinary tract. A variety of chemical and automated methods have been tried for the detection of the negative specimens, but none has yet been generally accepted as sufficiently reliable for its purpose. Recently. it has been reported that the finding of negative results in all of three chemical tests for nitrite, blood and protein, performed by a rapid automated dip-strip method (N-Labstix. Ames), predicts the absence of bacteriuria in about half of the culture-negative specimens. which may then be discarded.
Significant bacteriuria
The specimen most easily and therefore most commonly collected is mid-stream urine (MSU).
Although the greater part of the urinary tract is devoid of a commensal flora and bladder urine ii an uninfected person is free from bacteria, a specimen of spontaneously voided urine is to be contaminated with some commensal bacteria from the urethral orifice and perineum, particu¬larly in females, even when the most careful precautions are taken to prevent such contami¬nation. As these contaminating commensals include the very bacteria, such as E. coli and S. saprophyticus, which are the commonest organ¬isms to infect the urinary tract, the simple demonstration that bacteria of one of these species are present in the sample of urine is not poof that it has been derived from an infection in the urinary tract.
Proof of a urinary tract infection requires the demonstration that the potential pathogen is present freshly voided urine in numbers greater than those likely to result from contamination from the urethral meatus and its environs . The observations suggested that this number, taken to indicate significant bacteriuria , is about 100 000/ml. In true infections, in the absence or chemo therapy , the number of the infecting bacteria is likely to be at least as great as this. Accordingly a quantitative method of culture is adopted to estimate the number of viable bacteria in the specimen.
When properly collected specimens of urine are examined, contamination accounts for less than 104 organisms/ml and usually for less than 103/ml. Counts due to contamination are variable and the colonies often of diverse species. Specimens from urinary tract infections almost always contain more than 104 organisms/ml, usually more than 105/ml and often up to 108/ml. These high counts, which are fairly constant in serial specimens from the same patient, reflect bacterial multiplication in the urine in vivo and are accepted as indicating significant bacteriuria. The growth obtained in such cases usually represents a single infecting species, though some infections with two species. e.g. E. colil and S. faecalis, are encountered.
Significant bacieriuria (count >105/ml in a carefully taken and promptly examined sample) may sometimes occur in the absence of symp¬toms and pyuria in patients who subsequently develop symptoms of urinary tract infection. e.g. in pregnancy. The detection of such asymp¬tomatic bacteriuria is of value for there is good evidence of its association with the development of pyelonephritis in some patients.
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LECTURES ON APPLIED CLINICAL MICROBIOLOGY (ASIN: B004Y0XF54)